1. Field of the Invention
The present invention relates to surface acoustic wave filter devices used as band pass filters in mobile phones and other electronic apparatuses. More particularly, the present invention relates to surface acoustic wave filter devices having a balanced-unbalanced conversion function and substantially equal input and output impedances.
2. Description of the Related Art
Recently, with the miniaturization and weight reduction of mobile phones, the development of composite components having composite multiple functions has been underway in addition to the reduction of the numbers of included components and the miniaturization thereof. Under such circumstances, more and more research has been conducted to produce a surface acoustic wave filter device having a balance-unbalance conversion function, commonly referred to as a balun function, as a surface acoustic wave filter device used in the RF stage of a mobile phone. Such a surface acoustic wave filter device has been incorporated in a mobile phone for use in the GSM system and other cellular phone systems.
For example, Japanese Unexamined Patent Application Publication No. 9-205342 describes a surface acoustic wave filter device having the balance-unbalance conversion function.
FIG. 18 schematically shows a plan view illustrating the electrode structure of an example of a longitudinally-coupled resonator-type surface acoustic wave filter device having a balance-unbalance conversion function. In a surface acoustic wave filter device 100, input impedance and output impedance are substantially equal and a balance-unbalance conversion function is provided. On a piezoelectric substrate, three interdigital transducers (IDTs) 102 to 104 are arranged along a surface acoustic wave propagating direction. Outside the area where the IDTs 102 to 104 are arranged, reflectors 101 and 105 are arranged along the surface acoustic wave propagating direction. The IDTs 102 and 104 are commonly connected with each other and are connected to an unbalanced signal terminal 106. Each edge of the IDT 103 is connected to a first balanced signal terminal 107 and a second balanced signal terminal 108.
In the filter device having the balance-unbalance conversion function, regarding the in-passband transmission characteristics between the unbalanced signal terminal and the first balanced signal terminal and the in-passband transmission characteristics between the unbalanced signal terminal and the second balanced signal terminal, the amplitude characteristics need to be equal and one of the filters needs to be 180 degrees out-of-phase with respect to the other filter. Additionally, out of the pass band, the amplitude characteristics and the phase characteristics need to be equal.
The amplitude balance and the phase balance are expressed as below, when the filter device having the balance-unbalance conversion function is regarded as a device having three ports, and for example, an unbalanced input terminal is a port 1 and first and second balanced output terminals are a port 2 and a port 3.
Amplitude balance=|A|. In this case, A=|20logS21|xe2x88x92|20logS31|.
Phase balance=|Bxe2x88x92180|. B=|∠S21xe2x88x92∠S31|.
S21 represents a transmission coefficient from the first port to the second port. S31 represents a transmission coefficient from the first port to the third port. The symbol A represents the difference between a decibel value of the S21 and a decibel value of the S31.
Ideally, in the pass band of the filter device, the amplitude balance is 0 dB and the phase balance is 0 degree. Out of the pass band thereof, the amplitude balance is 0 dB and the phase balance is 180 degrees.
However, in the surface acoustic wave filter device 100 shown In FIG. 18, the balances are not ideal and insufficient. The reason for this is that since the bridging capacitance between the IDT 103 and the IDTs 102 and 104 at each side is added to the balanced signal terminal 107 and a capacitance is inserted between the balanced signal terminal 108 and a ground potential, the balanced signal terminals 107 and 108 have different parasitic capacitances. Thus, due to the difference between the parasitic capacitances, the balances, particularly, the balances out of the pass band are lost and thereby attenuation out of the pass band decreases.
In order to overcome the problems described above, preferred embodiments of the present invention provide a surface acoustic wave filter device having a balance-unbalance conversion function that is arranged such that the balances out of the pass band are greatly improved and reduction in attenuation out of a pass band is prevented. Other preferred embodiments of the present invention provide a communication apparatus including such a novel surface acoustic wave filter device.
According to a preferred embodiment of the present invention, a surface acoustic wave filter device has equal input impedance and output impedance and includes an unbalanced signal terminal, first and second balanced signal terminals, 2nxe2x88x921 first surface acoustic wave filters connected between the unbalanced signal terminal and the first balanced signal terminal, and 2nxe2x88x921 second surface acoustic wave filters connected between the unbalanced signal terminal and the second balance signal terminal, where n is an integer equal to 1 or more. In this filter device, one of the input and output impedances of each of the first and second filters is approximately four times the other impedance and the second surface acoustic wave filters are 180 degrees out-of-phase with respect to the first surface acoustic wave filters.
In addition, each of the first and second surface acoustic wave filters may have one or more interdigital transducers arranged in a direction in which a surface acoustic wave propagates and at least one of the interdigital transducers may be halved in an electrode-finger interdigitating widthwise direction to define first and second IDT sections which are connected in series with each other.
Furthermore, each of the first and second surface acoustic wave filters may have a structure in which a preliminary surface acoustic wave filter having substantially equal input impedance and output impedance is constructed in such a manner that at least one interdigital transducer is halved in an electrode-finger interdigitating widthwise direction to define first and second interdigital transducer sections.
The preliminary surface acoustic wave filter may be a longitudinally-coupled resonator-type surface acoustic wave filter. In addition, the longitudinally-coupled resonator-type surface acoustic wave filter may have three interdigital transducers arranged in a surface acoustic wave propagating direction and a central interdigital transducer or interdigital transducers at both sides may be halved in the electrode-finger interdigitating widthwise direction to define the first and second interdigital transducer sections.
Furthermore, each of the first and second surface acoustic wave filters may have one or more interdigital transducers and at least one interdigital transducer may be halved in a surface acoustic wave propagating direction to define first and second interdigital transducer sections.
In addition, each of the first and second surface acoustic wave filters may have a structure in which a preliminary surface acoustic wave filter having substantially equal input impedance and output impedance is constructed in such a manner that at least one interdigital transducer is halved in a surface acoustic wave propagating direction. Although the preliminary surface acoustic wave filter is not specifically designated, preferably, it is a longitudinally-coupled resonator-type surface acoustic wave filter.
The preliminary longitudinally-coupled resonator-type surface acoustic wave filter may have three interdigital transducers and a central interdigital transducer may be halved in the surface acoustic wave propagating direction.
In addition, preferably, either the first interdigital transducer sections or the second interdigital transducer sections may be connected to a ground potential. With this arrangement, one of the input impedance and the output impedance of the SAW filter between the balanced terminal and the unbalanced terminal is preferably approximately four times the other impedance thereof.
In addition, each of the first and second surface acoustic wave filters may have a structure in which a surface acoustic wave filter having a plurality of interdigital transducers is constructed in such a manner that at least two of the interdigital transducers are connected in series.
In addition, each of the first and second surface acoustic wave filters may have a structure in which a preliminary surface acoustic wave filter having substantially equal input impedance and output impedance is constructed in such a manner that at least two of the interdigital transducers are connected in series. Furthermore, preferably, the preliminary surface acoustic wave filter is a longitudinally-coupled resonator-type surface acoustic wave filter.
In this case, the longitudinally-coupled resonator-type surface acoustic wave filter may have three interdigital transducers and the interdigital transducers arranged at both sides in a surface acoustic wave propagating direction may be connected in series.
According to another preferred embodiment of the present invention, a communication apparatus includes the surface acoustic wave filter device of other preferred embodiments of the present invention defining a band pass filter.
Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments with reference to the attached drawings.